Presently, ambient vibration energy can be harvested and converted to useful electrical energy. The electrical energy can be used for powering low-power electronic systems such as sensors, MEMS devices or implantable medical electronics. Vibration energy can be harvested using various types of transducers like electromagnetic, electrostatic or piezoelectric. Electromagnetic device based harvesters employ a coil to tap energy from a moving magnetic field produced by magnets, which move in the ambient vibration. In electrostatic energy transducers, vibration leads to a relative parallel motion between an electret and nearby metallic surface. This leads to induced counter charge on the metal, with the charge simply following the relative motion of the electrets. This produces an electric current. Piezoelectric harvesters work by stressing a piezoelectric material in accordance with the vibration and tapping the generated electric charge from the material.
The electrical energy from any vibration energy harvester device is converted to a load usable form by an interface circuit, which is connected between the device and its load. The interface circuit consists of a rectifier-filter section for conversion of input ac to dc. The diodes in the rectifier conduct with a voltage drop, leading to power loss. The power loss due to diode-drop is a significant fraction of the overall harvested power which is in the range of 10-100 μW. It is therefore required for the diodes to conduct with minimum voltage drop.
There is the need of a diode equivalent circuit, which has a very low forward conduction drop, blocks any reverse conduction, can be used as a standalone diode able to replace the four diodes in full wave bridge rectifier (FBR), does not use an additional external power supply and is powered directly from input signal and not from harvested power, so as to use the available input power instead of the extracted output power and so that it consumes no standby power. Also, the diode equivalent should not demand additional off-chip components and the control circuit used should draw minimum power.
Conventional techniques have used synchronous rectifiers. A synchronous rectifier is a MOSFET which is controlled such that it conducts in the deep triode region in one direction and goes into sub threshold region i.e. OFF state in the other direction. Many of these synchronous rectifiers require an external power supply or a second piezoelectric patch for powering up a comparator, while one of the existing techniques to reduce diode drop requires an additional phase-shifted input for full wave rectification. The prior art passive full wave implementations and passive standalone diodes provide low diode drop without the use of external power supply. However, a few of these require additional off-chip capacitors and most of these are less power efficient due to transistors operating in saturation region instead of the cut off region and the consequent reverse conduction or due to continuous conduction of control circuit components such as diodes and resistors. A MOSFET based full wave rectifier followed by a diode is a prior art initiative to block reverse conduction. This technique and other passive rectifier implementations have used the output power instead of the input power, to power its comparator and other control circuit elements. This leads to consumption of standby power. The issue has been taken care of, in another known prior art with an input powered full wave rectifier and active diode. However, this technique and the prior art gate cross-coupled topologies of MOSFET based full wave bridge rectifier, are specific circuits for full wave bridge rectifier and do not come to use as a standalone low drop diode. Also, a few prior art passive half and full wave rectifier techniques with reduced diode drop do not fit in to be used in a full wave bridge rectifier.
Hence, there is the need of a solution for a low drop diode which can overcome the disadvantages of the existing methods and meet the requirements of being an energy efficient, signal powered and standalone diode capable of replacing any rectifier diode. Further, the solution should use minimal external hardware.